DEVELOPMENTAL ABILITY OF ZONA-FREE MOUSE EMBRYOS IS INFLUENCED BY CELL ASSOCIATION AT THE 4-CELL STAGE

Zona-free embryos at the 4-cell stage were classified into four types according to the total points of contact between the 4 blastomeres. Ty pes A, B, C, and D had 3, 4, 5, a nd 6 points of contact, respectively . Embryos of these types were cultured individually in 20 mu l of Whit ten's medium. Blastocysts derived from types A and D embryos at the 4- cell stage were transferred and observed on Day 17 of gestation. The n umber of live fetuses was significantly lower for type A than for type D embryos (p < 0.05). In spite of the removal of the zona pellucida, both types A and D blastocysts had cell numbers similar to those in zo na-intact control embryos at 96 h after insemination. However, type A embryos had fewer (p < 0.05) inner cell mass (ICM) cells than did eith er the zona-intact control or type D embryos. Type D embryos did not h ave significantly fewer ICM than did the zona-intact control. Our data show that the cell association of zona-free embryos at the 4-cell sta ge influences differentiation of cells within the ICM and thus affects subsequent embryonic development. We suggest that one role of the zon a pellucida after fertilization, in addition to protecting the preimpl antation embryo, is to maintain the cell arrangement of 4-cell-stage e mbryos to ensure subsequent successful cell lineage.